1. Field of the Invention
The present invention relates to a base station that communicates with a mobile station by using radio signals, and to a communications method of the base station.
2. Description of the Related Art
In a multiple access scheme employed in a mobile communications system, a plurality of mobile stations share the same radio transmission channel while making communications. As such a multiple access scheme, the following ones have heretofore been known: a Code Division Multiple Access (hereinafter referred to as CDMA) scheme, a Wideband CDMA (hereinafter referred to as W-CDMA) scheme, a Time Division Multiple Access (hereinafter referred to as TDMA) scheme, a Frequency Division Multiple Access (hereinafter referred to as FDMA) scheme, and the like.
In a mobile communications system that employs, for example, the CDMA scheme among the above-described multiple access schemes, the same frequency band is shared among a plurality of radio waves for communications. Such a mobile communications system is configured so that each of the radio waves for communications can be discriminated according to a spreading code which is a random code sequence.
Particularly, in the case of a mobile communications system of the W-CDMA scheme, it is required that a spreading code used for the mobile communications system have a code orthogonality enough to reduce mutual interferences among spreading codes different from one another, in addition to high detection and identification characteristics for the purpose of achieving high quality communications with low transmission power. For this reason, the number of allocatable spreading codes is less than the number of uplinks. In addition, although each of the spreading codes is repeatedly used among all the cells for effective use, a different code is used in addition to the spreading code for each of the base stations, for the purpose of preventing interferences among base stations (among cells). Moreover, in the case of a mobile communications system of the W-CDMA scheme, a spreading code with which user's data are actually spread in a wideband is referred to as a channelization code, and a code which is superimposed for each base station (cell) to prevent interferences among cells from occurring is referred to as a downlink scrambling code (refer to Japanese Patent Application Laid-open Publication No. Hei 11-215552).
In addition, in such a mobile communications system, a mobile station detects a base station, with which the mobile station is to communicate, on the basis of downlink scrambling codes informed from base stations to the mobile station. Moreover, in such a mobile communications system, each base station stores in advance, in a table (hereinafter referred to as a surrounding zone table), parameter information for radio communications, such as downlink scrambling codes, which are used by other base stations located in the surrounding zone of the base station, and thus the base station informs a mobile station of the downlink scrambling code through its own control channel. In addition, when a mobile station changes a base station with which the mobile station communicates due to the movement (for example, a handover is conducted), the mobile station detects a base station earlier, with which the mobile station is to communicate, on the basis of the downlink scrambling code informed prior to the moving. For this reason, in such a mobile communications system as described above, for example, in a case where a base station is newly established, it is necessary to identify other base stations (cells) located in the surrounding zone of the newly established base station (a cell), and concurrently store scrambling codes of the other base stations thus identified in a surrounding zone table of the newly established base station.
In this case, all the scrambling codes used by the mobile communications system may possibly stored in the surrounding zone table of the newly established base station no matter whether or not a base station is located around the newly established base station. However, the number of scrambling codes which can be stored in a surrounding zone table is limited (for example, 32) depending on a mobile communications system. Accordingly, in some cases, when scrambling codes are set in a surrounding zone table of a newly established base station, it is necessary to identify the other base stations located in the surrounding zone of the newly established base station on the basis of positional relationships on the topology between the other base stations and the newly established base station.
However, in this case, the actual radio range of the base station is sometimes different from a radio range on the topology due to the influence of, for example, structures and like. For this reason, the setting in the surrounding zone table may not be accurate. Specifically, on the basis of the actual radio ranges, scrambling codes of base stations that are located in the surrounding zone may not be stored in the surrounding zone table, but scrambling codes of base stations that are not located in the surrounding zone may be stored in the surrounding zone table. Therefore, for the purpose of more accurately storing the scrambling codes of other base stations in the surrounding zone table, a method in which radio signals are measured on the spot, or the like, has been used.
However, in a measuring operation on radio signals on the spot, it is necessary to measure radio signals in a wide radio range, leading to a large workload. Moreover, it is difficult to identify base stations located in the surrounding zone by accurately measuring radio range.
The present invention has been made in consideration of the above-described problems. An object of the present invention is to provide a base station and a communications method, in which parameter information for radio communications informed by the own base station can be set with a high accuracy, and which reduce workload at the time of the setting.